Please enable JavaScript.
Coggle requires JavaScript to display documents.
John Binner - Week 7 - Coggle Diagram
John Binner - Week 7
Rocket Nozzles
Almost all rocket engines use a de Laval nozzle. Shaped like a bell with the combustion chamber behind the pinched neck, the goal is for the exhaust to reach the highest possible velocity before it exits the engine. In normal circumstances its impossible to accelerate gases past the speed of sound by pushing them forward faster - the pressure increases faster than the velocity - known as choking.
In de Laval nozzles, the gases choke at the neck and then expand as they move into the nozzle, allowing them to accelerate past the speed of sound. The nozzle's shape is thus designed to maximise this acceleration by maximising the expansion of the gases.
Acceleration is maximised when the flow is perfectly expanded. A perfectly expanded flow is where the exit pressure exactly equals the ambient pressure. Unfortunately, this means means that nozzle optimised for launch will be much less optimally efficient at very high altitude and vice versa. A further problem is that the diameter of the de Laval nozzle increases as the material is ablated away due to the presence of hot gases (the choking is therefore decreased).
The ablation of material can be minimised by circulating cryogenic liquid around the fuel nozzle to cool it. This enables the use of a wide range of materials
-
-
-
Niobium - can be used uncooled due to melting point of 2468 C, but is extremely expensive, so usually has to be retrievable.
Since it is at the throat where gases are at their hottest, fastest and most ablative, one potential solution is to use an ablation resistant insert.
Ultra high temperature ceramics are ceramics that can withstand temperatures in excess of 2000 C (e.g. HfC, HfB2, ZrC, ZrB2). However, like all ceramics, they are brittle, hence the idea of reinforcing them with carbon fibres to form UHTCMCs.
Fibre layers can have different lay ups - stacked 2D layers, needled '2.5' D layers, or woven 3D layers
Fibre layers are then either infiltrated with a slurry or a hot melt (need to make sure that fibres can withstand temperatures of melt).
Finally, fibre layers can either be sintered (to densify the matrix), pyrolysed (to convert a polymer based matrix) or CVI'd to fill a porosity with a matrix. (CVI = chemical vapour infiltration)
Chopped fibres can also be incorporated into slurry for reinforcement, but does not provide aligned properties
Arc jet testing
-
Heat fluxes up to 150 MW m^-2 - ultra high temps, supersonic / hypersonic speeds
Advantages - most realistic test for aerospace applications, large samples can be tested
Disadvantages - not cheap, relatively few systems in the world
-
-
Hypervelocity flight
-
Primary issue is control of flight direction - sharp leadings edges are required. These lead to very significant heating - sometimes up to 3000 C.
ZrB2 can withstand up to 2500 C, HfB2 can handle up to 3000 C. Unfortunately, HfB2 is twice as dense and 10 times more expensive than ZrB2
Although UHTCs can take these temperatures, they can't take rapid changes in temperature, so we need to have UHTCMCs
Self Heating testing
-
-
Disadvantages - hottest temp is down the centre of the bar (no chance of reactions with the atmosphere) so very difficult to know the heat flux, as there is no gas flow
-
-
UHTCMCs
Processing routes
Conventional routes
Advantages
-
-
Possible to add fibre coatings to optimise the fibre/matrix properties (e.g. weak interface composites)
-
-
Reaction mould injection
Wetting (penetration of ceramic slurry into metallic preform) is very critical for the infiltration process.
-
-
-
-
Points regarding testing
-
Cf-HfC tends to result in spallation; the CO2 formed tends to push off the oxide layer, whilst the B2O3 formed from HfB2 doesn't.
HfB2 will yield composites that can withstand 3000 C for several minutes. By comparison, ZrB2 is an order of magnitude cheaper, is approx. 50 % less dense and will withstand 2500 C for several minutes.
SiC additions to either of these can improve the oxidation resistance, but only up to 1700-1800 C
-